During the reaction of methyl chloride with silicon a mixture of chlorosilanes is produced. These are normally separated by distillation. Two of the largest volume chlorosilanes are dimethyldichlorosilane and methyltrichlorosilane. In order to prepare satisfactory siloxane polymers from dimethyldichlorosilane it is sometimes necessary that the methyltrichlorosilane content of the dimethyldichlorosilane be less than about 500 parts per million. The boiling points of these materials are sufficiently close that distillation columns of 200 stages or more are required to satisfactorily separate these materials in commercial operation. Consequently at the present time a large capital investment is required in order to install these columns and it would be highly desirable to reduce this capital investment. Also, a large column generally requires more energy to operate than a smaller column. Careful fractional distillation is also employed by the organosilicone industry to separate other close-boiling chlorosilanes.
Thus one object of this invention is to provide a method of separating dimethyldichlorosilane and methyltrichlorosilane which uses smaller distillation columns and requires less energy thereby reducing the capital investment necessary in basic chlorosilane plants. Another object of this invention is to provide a method of separating close-boiling chlorosilances which uses smaller columns and requires less energy. Still another object of this invention is to provide a method of separating chlorosilanes which will increase the capacity of existing distillation columns of basic chlorosilane plants. Other objects of the instant invention will be apparent from this specification to those skilled in the art.
U.S. Pat. No. 3,007,956 (issued Nov. 7, 1961) predicts that one might carry out an extractive distillation of mixtures of chlorosilanes in the presence of certain dinitriles such as adiponitrile or glutaronitrile. Specifically, the patent teaches in Example 4, that, based upon beta/beta' values obtained from mixtures of dimethyldichlorosilane and methyltrichlorosilane in adiponitrile as compared to the vapor pressure of each alone, "those skilled in the art would know . . . that adiponitrile would be a suitable solvent . . . in accordance with the procedures of extractive distillation".
Sivtsova et al., J. Appl. Chem. USSR, 38, 2549 (1966), hereafter referred to a Sivtsova I, also discusses the problem of separating certain chlorosilanes by distillation. They measured the relative retention times of various chlorosilane mixtures with some 31 solvents on a gas chromatograph and predicted, based on relative retention times, that the materials would function as separating agents. They repeated the experiments with certain of the materials at different temperatures as shown in Table 2, p. 2551 of Sivtsova I. Among the solvents employed in Tables 1 and 2 is sulfolane (tetrahydrothiophene-1,1-dioxide). In no case did Sivtsova I carry out an actual distillation with any of the solvents nor did they obtain liquid-vapor equilibrium data.
Liquid-vapor equilibrium data at least are essential to determine whether a particular solvent, which gives promising relative retention times by gas chromatography, will actually enhance separation of the materials in question. For example, relative retention time studies alone do not predict the formation of azeotropes during actual distillation nor do they preclude the formation of a "pinch". A "pinch" is where the relative volatility approaches but is not exactly equal to 1.00 as opposed to an azeotrope where the relative volatility is exactly equal to 1.00. The formation of either an azeotrope or a "pinch" will make separation by distillation impossible from a commercial standpoint.
The inadequacy of the data in Sivtsova I is acknowledged in Sivtsova et al., J. Appl. Chem USSR, 39, 1908 (1967), hereafter referred to as Sivtsova II, (a subsequent publication) which admits that the investigations described in the earlier publication(Sivtsova I) "cannot give a full quanitative assessment" of the extractive distillation process. Sivtsova II further states that "liquid-vapor equilibrium data . . . are necessary for extractive distillation process calculations". In another subsequent publication, Sivtsova et al., J. Appl. Chem USSR, 41, 447 (1969), labeled Sivtsova III, the authors state that liquid-vapor equilibrium studies "are necessary for calculations relating to extractive distillation of these materials." Sivtsova II uses beta-beta' dichloroethylether and Sivtsova III employs ethylmonochloroacetate as the extractive solvent. Thus, in all their publications subsequent to Sivtsova I, no mention is made of sulfolane. It also does not appear in their Russian Pat. Nos. 165,445 and 275,054 appearing in Soviets Inventions Illustrated, June 1965, and March, 1971, respectively.
The reason for the omission of sulfolane in spite of the relative retention times shown in Table 2 of Sivtsova I, may be found on page 2549 of that publication. There the authors state, in referring to the extractive distillation use of the dinitriles of U.S. Pat. No. 3,007,956, that "these substances are extremely difficult to use because of their excessively high boiling points. For example, adiponitrile boils at 295.degree. C. and malonic dinitrile boils at 220.degree. C." Sulfolane boils at about 285.degree. C. at atmospheric pressure.
There are other reasons why solvents showing high relative volatility coefficients may not be suitable for commercial separation. First, the solvent may decompose under prolonged use or it may react with chlorosilanes or both. Second, the toxicity of the proposed solvent may be too high for use in commercial operation especially in the present age of environmental concern.